improve error handling for deleted users in course assignment

[AllanOXDi]

Summary

Please see #14209

Before

deleted.mp4

After

References

#14155

Reviewer guidance

See #14209

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.19.2.dev0_git.20260219141521.pex
Windows Installer (EXE)	kolibri-0.19.2.dev0+git.20260219141521-windows-setup-unsigned.exe
Debian Package	kolibri_0.19.2.dev0+git.20260219141521-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.19.2.dev0+git.20260219141521.dmg
Android Package (APK)	kolibri-0.19.2.dev0+git.20260219141521-0.1.6-debug.apk
Raspberry Pi Image	kolibri-pi-image-0.19.2.dev0+git.20260219141521.zip
TAR file	kolibri-0.19.2.dev0+git.20260219141521.tar.gz
WHL file	kolibri-0.19.2.dev0+git.20260219141521-py2.py3-none-any.whl

[marcellamaki]

hi @AllanOXDi -- can you please retarget this to develop since it will go there due to the new strings? thank you 🙏

my apologies, @AllanOXDi, I was reading this too quickly this morning and didn't realize that you had split the work into two distinct PRs and targeted just the courses one here. That was really thoughtful, and I'm sorry that I missed it! 0.19 is appropriate for this. I've updated accordingly. sorry for the confusion.

QA note: Allan and I discussed strings and for now they are "working" strings to convey the general behavior. this will still go through UX writing review for 0.20 :)

[AlexVelezLl]

Given that catching backend error messages will require some more design and decisions, we have agreed to keep this PR as is, and try to standardize our API error handling in a follow-up PR. So, this should be ready for QA review!

[pcenov]

Thanks @AllanOXDi - I confirm that attempting to assign a course to a deleted user results in seeing the following message: There was a problem saving this course. One or more selected users no longer exist in this facility. Please refresh the page and try again.
A note that we are still showing the generic error message Sorry! Something went wrong, please try again. if I try to assign the course to a deleted class or a group but my understanding is that this is good enough for the time being.